Optical frequency combs 1-3 provide equidistant frequency markers in the infrared, visible and ultra-violet4,5 and can link an unknown optical frequency to a radio or microwave frequency reference6,7 (citations refer to the listing of references at the end of the present description). Since their inception frequency combs have triggered major advances in optical frequency metrology and precision measurements6,7 and in applications such as broadband laser-based gas sensing8 and molecular fingerprinting9. Early work generated frequency combs by intra-cavity phase modulation10-12, while to date frequency combs are generated utilizing the comb-like mode structure of mode locked lasers, whose repetition rate and carrier envelope phase can be stabilized13.
Conventional techniques for generating optical frequency combs generally may have disadvantages in terms of complex optical setups and costly control thereof.
Optical microcavities21 are owing to their long temporal and small spatial light confinement ideally suited for nonlinear frequency conversion, which has led to a dramatic improvement22 in the threshold of nonlinear optical light conversion by stimulated non-linear processes23 such as Raman scattering. In contrast to stimulated gain, parametric frequency conversion24 does not involve coupling to a dissipative reservoir, is broadband as it does not rely on atomic or molecular resonances and constitutes a phase sensitive amplification process25, making it uniquely suited for tunable frequency conversion. In the case of a material with inversion symmetry—such as silica—the elemental parametric interaction involves four photons26 and is also known as hyper-parametric interaction27 (or modulation instability in fiber optics26). The process is based on four-wave mixing among two pump photons (frequency ωP) with a signal (ωS) and idler photon (ωI) and results in the emergence of (phase coherent) signal and idler optical sidebands from the vacuum fluctuations at the expense of the pump field. The energy conserving nature of the process (ωI+ωS=2ωP) poses stringent conditions on the amount of cavity dispersion that can be tolerated to observe parametric interactions, while momentum conservation are intrinsically satisfied for signal and idler modes. Indeed, it has only recently been possible to observe these processes in crystalline CaF2 and silica microcavities15,16, in which all-resonant parametric oscillation is achieved owing to nonlinear mode pulling generated by self-phase and cross-phase modulation15.
Conventional techniques of using optical microcavities for non-linear light conversion have a disadvantage with regard to the strongly limited number of sideband frequencies generated in the resonator. Accordingly, the optical microcavities could not be used for generating a frequency comb. Generation of increased number of sideband frequencies was excluded due to the effect of dispersion in the cavity15.